Microwave band reflection filter



June 6, 1950 w. D. LEWIS ET AL 2,510,238

MICROWAVE BAND REFLECTION FILTER Filed Dec. 5, 1947 FIG.

(TA f? I 7 1 1 I i I F IG. 2

sq? FIG. 3 3/0 A 7' TORNE V with the shorter "sides. practice beadhered'to in connection with the -illustrativ'e embodiments describedbelow.

Patented June 6, 1950 MICROWAVE. BAND REFLECTION FILTER Willard D.Lewis, Little Silver, and Le Roy 0. .Tillotson,Holmdel, N. J assignorsto Bell Telephone Laboratories, Incorporated, New York, N. Y., acorporation of New York Application December 5, 1947,'Serial No. 789,986

4 Claims. 1 This invention relates to a filter for use at ultra-highfrequencies and microwave frequencies in transmission systems employingwave guide transmission lines. A principal object of the'invention is toprovide simple compact filters which can be readilyintegrated into waveguide transmission systems.

Another objectis to provide filters of the abovementioned characterwhich are readily adjustable with respect to resonant frequencies andimpedance level.

A further object is to provide filters of the above-mentioned characterwhich are well suited for use as band or channel reflecting filterswhere the band or channel of frequencies to be reflected is relativelyvery narrow, i. e., where the ratio of the'band or channel width incycles to the mid-frequency of the band or channel in cycles is in theorder of afew per cent or less.

Other and further objects of the invention will become apparent duringthe course of the following description of a preferred embodiment of theinvention and 'from the appended claims.

The filters of the invention are primarily intended for usein systemsemploying wave guides to transmit TE1,o waves as defined, for example,at pages 316 to 322 of Electromagnetic Waves by S. A. Schelkunofi,published by D. Van Nos trand Company, Incorporated, New York City,1943. Where a wave guide of rectangular crosssection, in which onecross-sectional dimension is shorter than the other,'is used, it is theusual practice to employ TE1,0 type waves with the electri'cal'vector(E-plane) of the wave parallel It is intended that this The basicstructuralunit-of these filters is a "resonatofwhich comprises a'slenderrod placed centrally within the wave guide perpendicularly tothedirection of the electric vector of'the wave,

one end of-the rod connecting directly'to the side wall of the wvaveguide, the other end of the fiO'd being capacitatively coupledto acorresponding point on'the oppositeside wall of'the wave guide. A smalldisc mounted on the free end ofthe rod and a'tshort stud memberextending:from the nearest adj acent point on the opposite sidewall of'theguide'providethe capacitative coupling. As the resonator would normallyhave no coupling to the "wave passing through the guide, a coupling isestablished byinserting an adjustable stud member through a wall of the.guiderin :the :vicinity :of :the resonator and perpendicularly'to'the'rod. This distorts the field in-the guide and thus affords acoupling of the wave to the resonator rod.

Two or more resonators can be employed spaced along the wave guide atquarter wavelength-intervals of the mid-frequency of the'band to bereflected.

The principles of the invention will be more readily understood inconnection with the description of specific illustrative embodimentsthereof described in detail below and illustrated in the accompanyingdrawings in which:

Fig. 1 shows a side view of a band or channelrefiecting filter of theinvention;

Fig. 2 shows an end view of the same filter; and

Fig. 3 shows an equivalent electrical schematic diagram of the filter.

A particular use of this specific filter is dis closed and described inthe copending application of W. D. Lewis (one of the applicants of thepresent application), SerialNo. 789,985, filed December 5, 1947 andassigned'to applicants asapplication of W. D. Lewis.

Assembled within this section of wave guide are, in the particularfilter shown by way of example, three resonators, each comprising aslender rod 104, terminated at one end by a'small disc 106 andfirmlyattached at the other end to an E-plane wall of the guide at a point onthe longitudinal center-line of the wall. As

shown clearly in Fig. 2, the rod of each resonator is parallel with theH-plane dimension oi the wave guide, centrally located between theH-plane sides of the guide, and extends nearly across the full H-planedimension of the'guide. A screw H18, having a diameter substantiallyequal to'the diameter of disc 106 is threaded through the E-plane wallof the wave guide Just opposite the point at whichrod TM is 'fastened tothe other E-plane wall. The screw is .pro-

vided with a lock nut H0 to hold it in place -w hen'it has been adjustedas described above.

As stated above, the structure shown in Figs'xl and 2'is to be used totransmit a type of wave having its electrical vector perpendicular tothe broader sides of the guide. In order to couple the rod 106 to such awave it is necessary to distort the wave form by inserting one or moremembers, such as screw H2 through either or both of the H-plane sides ofthe guide in the immediate vicinity of the rod 104. Increasing theextent to which screw H2 extends into the guide increases the distortionof the wave form and thereby increases the degree of coupling of thewave with rod 104. Increasing the coupling effectively increases thebroadness of the response of the resonator, or the frequency region overwhich the resonator affects the transmission. A lock nut H is providedto hold the screw at its desired position of adjustment.

The combination of rod 104, which contributes an inductive reactance,and disc 106 together with the inner end of screw 708, which contributea capacitative reactance, can be adjusted by turning screw 108 into orout of the wave guide to increase or decrease the capacity,respectively, to cause the combination to resonate at any frequencywithin the range being transmitted. At the frequency of resonance and atfrequencies closely adjacent above and below the resonant frequency, theresonator will cause reflection of the wave back in the direction fromwhich it 'enteredthe section of guide, while other frequencies will passfreely through the guide.

A smoother, or flatter, reflection band i ob- Itainedby addingadditional resonator structures like the one just described, spacedalong the guide by one-quarter wavelength of the mid-frequency of theband or channel of frequencies to be reflected, a maximally flat band isobtained by greatly increasing the number of resonators and tuning themall to the mid-frequency of the band. With'a relatively few resonators aslightly broader but less smooth (or flat) band can be obtained'byspacing some of the resonances near the edges of the hand. For a systemsuch as that described in the above-mentioned copending application ofW. D. Lewis, in which bands or channels 20 megacycles wide centeredabout specific frequencies in the neighborhood of 4000 megacycles, areto be reflected, three resonators, as indicated in Figs. 1 and 2, havebeen found to be In this instance, one resonator was tuned to resonanceat the mid-frequency of the band and the other two were adjusted toresonance near the upper and lower edges of the band, respectively. Toreflect broader frequency bands or channels, or to provide more uniformreflecting characteristics over any given frequency band or channel,additional resonators, spaced along the guide as described above, can beadded and adjusted at appropriate resonant frequencies and withappropriate effective coupling to providethe desired bandwidth anduniformity of refiection over the band.

The effective inductance of the rod at ultrahigh and microwavefrequencies is, of course, af-

fected by its diameter, as wel1 as its length. A rod diameter of a einch and a disc diameter of 1;; inch were found suitable with the waveguide dimensions and frequency range mentioned above, by way of example,and in the illustrative system of the above-mentioned copendingapplication of W. D. Lewis.

The equivalent electrical schematic diagram of the filter of Figs. 1 and2 is shown in Fig. 3. Coils 302 'to 304, inclusive, with theirassociated variable'condensers 305 to 30?, inclusive, respectively,represent the three resonators and the spacing along the line extendingfrom terminals 300, 3M

4 to terminals 3I0, 3| I, between resonators is 7/4.

A flange 102 is provided at each end of the section of wave guide tofacilitate joining wave guides of similar inside dimensions to the endsof this reflecting filter. Sections of the front flange in Fig. 2 arebroken away to show the screws I08 and H2 and their associated lock nutsmore clearly.

Those skilled in the art can readily devise many equivalent structureswith which to practice the principles of the invention. It is obvious,of course, that lengths of wave guide of square or of circularcross-section can be employed in constructing filters of the invention,the sole requirements being that the resonators are alignedperpendicularly with respect to the electrical vector of the wavepassing through the guide and that the coupling adjustment screws arelocated and aligned to produce a distortion of the field sufilcient toeffect the desired degree of coupling to the resonators. Any othermeansfor effecting a suitable distortion of the field can, of course, beemployed. Also, in the case of a round waveguide, the portion containingthe resonators can be arranged to be turned with respect to theremainder of the waveguide and the desired coupling established by thusturning the resonator slightly away from precise perpendicularity withrespect to the electric vector of the wave. The scope of the inventionis defined in the fol1owing claims.

What is claimed is:

1. A wave guide electrical filter, for use in wave guide transmissionlines transmitting waves having a definitely oriented electric vector,comprising a section of wave guide, a resonant structure within saidwave guide, said resonant structure comprising an electricallyconductive rod conductively coupled at one end to a first interior pointof said wave guide and capacitatively coupled at the other end to asecond interior point diametrically opposite to said first point withinsaid wave guide, said rod being oriented to have zero coupling to theelectrical wave energy normally transmitted through said wave guide andadjustable means projecting within said wave guide in the vicinity ofsaid rod and substantially perpendicularly thereto, for establishing acoupling between said rod and said electrical wave energy by distortionof the field in said guide in said vicinity.

2. The filter of claim 1 and means for adjusting the capacitativecoupling to said other end 01 said rod.

3. A wave guide electrical filter, for use in wave guide transmissionlines transmitting waves having a definitely oriented electric vector,comprising a section of wave guide, a plurality of resonant structureswithin said wave guide, said resonant structures each comprising anelectrically conductive rod conductively coupled at one end to a firstinterior point of said wave guide and capacitatively coupled at theother end to a second interior point, diametrically opposite to saidfirst point within said wave guide, said rod being oriented to have zerocoupling to the electrical wave energy normally transmitted through saidwave guide and adjustable means projecting within said wave guide in thevicinity of said rod and substantially perpendicularly thereto forestablishing a coupling between said rod and said electrical wave energyby distortion of the field in said guide in said vicinity, saidplurality of resonant structures being spaced longitudinally along saidsection of wave guide at intervals of 5 substantially one-quarterwavelength of a prede- REFERENCES CITED termined frequency within therange of frequen' The following references are of record in the cies tobe transmitted through said wave guide. file of this patent:

4. The filter of claim 3 and means associated with each rod of saidplurality of resonant struc- 5 UNITED STATES PATENTS tures for adjustingthe capacitative coupling of Number Name Date one end of the rod to saidwave guide. 2,104,915 Thompson Jan. 11, 1938 2,425,345 Ring Aug. 12,1947 WILLARD D. LEWIS. 2,432,093 Fox Dec. 9, 1947 LE ROY C. TILLOTSON.10 2,438,119 Fox Mar. 23, 1948

